1. Field of the Invention
The present invention relates to a satellite radiowave receiving device that receives a radiowave from a positioning satellite and obtains a satellite signal.
2. Description of Related Art
Heretofore, there has been a portable apparatus such as an electronic wrist watch, which includes a receiving device that receives a radiowave transmitted from a positioning satellite according to a global navigation satellite system (GNSS) including a global positioning system (GPS), and is capable of obtaining time information and position information. In the portable apparatus as described above, based on the obtained time information and position information, a displayed time can be corrected, and a time zone can be set.
A satellite signal to be outputted from the positioning satellite is subjected to spread modulation by using an intrinsic spreading code (pseudo noise) set for each positioning satellite, and is transmitted on the radiowave. In the event of receiving this radiowave, this received radiowave is subjected to despreading by using the spreading code set in the positioning satellite as a transmission source of the received radiowave concerned, whereby the satellite signal from this positioning satellite is decoded and obtained. Each positioning satellite moves on a predetermined orbit, and in the receiving device, which does not have orbit information of such positioning satellites, and does not grasp in advance which positioning satellite is in a visible state and whether or not it is possible to receive a radiowave therefrom, despreading processing is performed for the received signal in the received radiowave by spreading codes of all of the positioning satellites in a round robin manner, whereby the satellite signal is detected, and the positioning satellite from which the radiowave is receivable is identified. In such a way, capturing processing is performed.
For the capturing processing as described above, heretofore, a matched filter (sliding correlator) has been used, and moreover, pieces of the despreading processing by the spreading codes of the plurality of satellites are performed in parallel, whereby acceleration of the capturing processing is achieved. Furthermore, Japanese Patent Laid-Open Publication No. 2001-159670 discloses a technology, in which a plurality of memories performing reading and writing of data are provided, and the plurality of memories are allowed to read and write data in parallel of the despreading processing, which are to be performed simultaneously in parallel, whereby a delay in I/O processing for arithmetic operation processing is prevented.
Moreover, Japanese Patent Laid-Open Publication No. 2002-122655 (corresponding to US 2002/0004392 A1) discloses a technology, in which power supply periods and clock signal frequencies for a correlator performing the capturing processing and a microprocessor performing calculation processing for a position and a time based on obtained data are individually subjected to setting changes according to needs, whereby a power consumption is reduced.
The radiowave transmitted from each positioning satellite is received at a frequency different from a transmission frequency owing to the Doppler effect brought by the fact that the positioning satellite moves around the earth at a high speed. Hence, in the event of receiving the radiowave from the positioning satellite by the receiving device that does not have current position data and the orbit information of the positioning satellite, it is necessary to search such a reception frequency of the radiowave from the positioning satellite within a frequency range that can be changed owing to the Doppler effect. As a result, a time required for the capturing processing is increased in proportion to a width of the reception frequency for which the search is performed.
However, in terms of a ratio, a storage unit that stores such received data in the matched filter occupies a larger size in a capturing processing circuit in comparison with other portions for use in the capturing processing. Hence, when such storage units for processing the received data related to a plurality of reception frequencies in parallel are individually provided to attempt achievement of the acceleration of the capturing processing, a scale of a circuit that performs the capturing processing is increased.
The present invention is a satellite radiowave receiving device capable of shortening the time of receiving the radiowaves from the positioning satellites without enlarging such a circuit scale.